This was one powerful bomb.

In April, North Korea’s leader Kim Jung-un and South Korean President Moon Jae-in made the historic pledge to completely denuclearize the Korean peninsula. Since then, experts have questioned whether North Korea’s Punggye-ri test site was even in a stable enough condition to merit meaningful dismantling, and South Korean representatives have admitted that denuclearization will be especially difficult until they know all the facts about the north’s nuclear capabilities. The answers may lie in a revelatory study, published Thursday in Science, that describes a striking discovery made with a new method for nuclear surveillance.

In the paper, the researchers argue that there’s a better way to surveil for nuclear testing: via spaceborne monitoring. The scientists from Singapore, Germany China, and the United States write that space-based radar technology revealed that an underground nuclear test in September 2017 rocked Punggeye-ri with the power of a 5.2-magnitude earthquake. This explosion, which took place 450 meters under Mount Mantap, actually moved the mountain, pushing the surface of it outward by as much as 11 feet and left it 20 inches shorter. A cavity within the mountain, about the width of a football field, is also believed to have collapsed after the detonation.

This caving indicates that the damage to the site is more severe than what was reported to Jae-in by Jung-un, and likely can’t be used at all. Study author and Nanyang Technological University professor Sylvain Barbot, Ph.D., told the Wall Street Journal on Thursday that “a very large domain has collapsed around the test site, not merely a tunnel or two.”

The scientists used a combination of seismic recordings from global and regional networks, computer modeling, and before-and-after radar measurements of the ground surface from radar imaging satellites to determine the power of the blasts and the resulting destruction. About nine minutes after the blast, they discovered, an aftershock rattled the earth 2,300 feet south of the initial demolition.

The data reveals just how powerful North Korea’s nuclear arsenal actually is — and it really is quite powerful. The scientists write that the nuclear test within Mt. Mantap had a yield between 120 and 300 kilotons, which is ten times the strength of the bomb dropped by the U.S. on Hiroshima. This indicates that the bomb used was either a fusion bomb or fission bomb.

Mt. Mantap, the location of six nuclear bomb tests. The September test marked by red.

These findings were possible becaus the scientists thought to pair SAR (synthetic aperture radar) with the more traditional seismic analysis. Seismology has been used as nuclear weapon monitoring technology since the Atmospheric Test Ban Treaty of 1963, which sent many nations underground to test their weapons. Like earthquakes, nuclear explosions cause seismic waves, sometimes with vibrations so large that they are picked up by monitoring stations around the world. The realization that detonations set off these waves actually instigated the development of the Worldwide Standardized Seismograph Network, which is still used today to detect nuclear tests breaching the 1996 Comprehensive Test-Ban Treaty.

Seismologists have been monitoring North Korea since 1968 — especially since they intensified their underground nuclear weapon tests in 2006. The new technology can improve surveillance, said paper co-author and University of California, Berkeley researcher Douglas Dreger, Ph.D., in a statement released Thursday, explaining that SAR “has a unique role to play in monitoring explosions because it is direct imaging of the local ground surface, unlike seismology, where you can learn the nature of the source analyzing waves radiating outward from the event at distant stations.”

“This study demonstrates the capability of spaceborne remote sensing to help characterize large underground nuclear tests, if any, in the future,” co-author Teng Wang, Ph.D. of Nanyang Technological University, explained in the same statement. “While surveillance of clandestine nuclear tests relies on a global seismic network, the potential of spaceborne monitoring has been unexploited.”

President Donald Trump announced Thursday that he will be meeting Jung-un on June 12 for a face-to-face summit in Singapore. We’ll see then whether he brings up this study in his discussion of nuclear disarmament.